Glossary and definitions of terms

Vibration Technology

Vibration Technology, vibration isolation or insulation, vibration damping – whatever you want to call it, Bilz is your expert in this field. In order to ensure your personal background knowledge, we have compiled and explained some of the central terms and definitions around this topic.

 

Vibration or oscillation

Vibrations are temporal changes of state variables (e.g. path, acceleration, velocity, etc.) from an equilibrium value and a position of rest. There are different types (see also types of vibrations ), depending on the nature of their courses.

 

Types of vibrations

Periodic oscillation: In this type of oscillation the course can be accurately predicted because of the preceding regularity, such as in the harmonic oscillation which corresponds to the course of a sine function. It is also called deterministic vibration.

Stochastic Vibration: These are also called non-deterministic or random oscillations which are not predictable.

Impact/shock: The critical characteristics of an impact are its duration, distribution and intensity. Isolating an impact converts it from a short-term high-amplitude impulse into one which lasts longer but only carries a small fraction of the original force.

Structure-borne noise is caused by mechanical vibrations that propagate in a solid body. The airborne sound is audible.

Vibrations are periodic mechanical oscillations of bodies.

 

Vibration isolation

Vibration isolation means the decoupling of disturbance forces and vibrations. This effect can always be observed in both directions, i.e. from the engine to the environment as well as in the opposite direction (see also types of isolation). The term vibration damping (not to be confused with damping!) is often used as a synonym.

 

Damping D

is the physical property of an insulator to take away energy from the oscillation system. This energy is then converted into heat. The oscillations are thereby limited to an allowed measure. Damping D is a dimensionless size.

 

Load

The type of load is important when it comes to selecting and calculating the suitable vibration isolation. This load is composed of two parts: 1. static load caused by the weight of the machine and the workpiece, and 2. dynamic load caused by the movement of the vibration. Unit: Newton (N).

 

Amplitude

AmplitudeMaximum deflection of a harmonic oscillation from the position of rest. Peak amplitude (1). Peak-to-Peak (2). Root Mean Square amplitude (3), wave period (4)
Units:        (1), (2) und (3): meter (m)
(4): second (s)

 

Frequency f

is the measure of the number of oscillations per second. The unit is Hertz (Hz).

 

Natural frequency = resonance frequency

is the frequency at which the vibrating body oscillates freely about its position of equilibrium after a singular excitation.

 

Excitation or disturbance frequency

is the frequency at which the system is excited in the case of a forced vibration. Often it is equal to the engine speed or stroke rate.

 

Matching ratio

The ratio between excitation and natural frequency is called matching ratio. In general, it is true that the effectiveness of vibration isolation rises as the natural frequency of the isolator drops, i.e. as the ratio between the frequency of the vibration and the natural frequency of the isolator rises. Typically, the objective is to achieve a ratio of between 3 and 4.

 

Resonance / resonance amplification

If the natural frequency is close to the excitation frequency or even equal to (damping ratio = 1), it is called resonance. The oscillating system consisting of machine and isolator reacts with particularly large amplitudes. The vibration is not reduced, but enhanced (resonance amplification).

 

Actuators

are elements that convert an input signal into a different type of signal. It is mostly mechanical work that is converted. Thereby, they influence the control system actively and provide reference values.

 

Active / passive vibration isolators

Passive isolators show resonance amplification in the range of the natural frequency when excited. This resonance amplification depends on the damping of the isolators.
Active isolators generate a 180° phase-shifted opposing force (see also phase shift) when regulated appropriately, whereas the isolators function as actuators here. Thus, the resonance amplification is minimized in the range of the natural frequency of the isolators.
Isolators are also commonly referred to as vibration dampers.

 

Degrees of freedom (DOF)

are the possible directions of motion of a system. A solid body has six degrees of freedom, because you can move the body in three independent directions ( X , Y , Z ) and rotate it around each of the three axes.

 

Center of Gravity (COG)

The location of the center of gravity of a machine is important for static mounting and the selection of mounting points when it comes to vibration isolation.

 

Types of isolation

Source isolation (isolation of vibration emission): When a machine is mounted on vibration isolators, the vibration forces are reduced. Hereby also the environment, such as neighboring machines, buildings and people are protected from the disturbing forces.

Recipient isolation (isolation of vibration immission): In this case it is important to protect vibration-sensitive equipment (e.g. measuring machines) from disturbing ground vibrations.

 

Phase shift

The physical concept of phase shift means that two harmonic oscillations are phase-shifted if their cycle lengths coincide but not the timing of their zero-points. A phase shift of 180° (= antiphase) is used with active isolators (e.g. Active Isolation System™) in order to isolate vibrations and to minimize resonance amplification.

 

Direct isolation

Direct isolation means that vibration isolators are mounted directly below or in the machine, usually at the same place as existing mounting points. Direct isolation requires a sufficient rigidity of the machine bed and the base frame so that is does not twist due to the elastic machine mounting.

 

Indirect isolation

For machines with a rigidity that is not sufficient for direct isolation you need a rigidly designed intermediate structure which must be installed between the machine and the isolators, e.g. steel platform, cast iron plate or a block concrete foundation (foundation isolation).